There are several synonyms for the phrase "on call," some of which include "on standby," "on duty," "available," "ready to respond," "at the beck and call," and "on the alert." These synonyms all denote a state of readiness or availability to perform a task or respond to a request. Whether it's a medical professional on call for emergencies, a technician on standby to handle equipment malfunctions, or a customer service representative ready to assist customers at any time, being on call requires a certain level of responsibility and dedication to one's profession. Whatever the specific context may be, being on call implies a commitment to being available and responsive when needed.